1. Field of the Invention
The present invention relates to a signal processing apparatus in which transducers for exciting or receiving surface acoustic wave (hereinafter referred to as SAW) are formed on a piezoelectric substrate of a single crystal such as lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3), lithium borate (Li.sub.2 B.sub.4 O.sub.7), quartz, etc. or on a piezoelectric substrate in which a film such as a zinc oxide (ZnO) film, etc. is formed on a non-piezoelectric substrate so that filtering of high-frequency signals and making correlation between high-frequency signals are performed by means of converting electric signals into SAW signals and converting SAW signals into electric signals.
2. Description of the Related Art
An SAW correlator (convolver) shown in FIG. 1 is a typical example of a conventional signal processing apparatus using SAW. In the SAW correlator, transducer electrodes 4 and 5 for exciting SAW are formed on an upper surface of a piezoelectric substrate 1, and a uniform electrode 9 is formed between two transducers disposed in the center portion of the piezoelectric substrate 1. A uniform ground electrode 6 is formed under a lower surface of the piezoelectric substrate 1. Independent high-frequency signals are applied from independent electric terminals 2-1 and 3-1.
The high-frequency signals thus applied are converted into SAW signals, and the SAW signals are inputted into the uniform electrode 9 in the center portion from left and right sides. When the amplitude of each of the high-frequency signals is large at some extent, the member of the product of the high-frequency signals entering from the left and right sides is generated under the electrode 9 by the nonlinear effect of SAW. This member is integrated spatially and taken out as an output between the terminal 8 and the ground 7.
The signal processing apparatus shown in FIG. 1 can be used as a demodulator for spread-spectrum communication. That is, a spread-spectrum signal received by an end device is inputted to the terminal 2-1. A replica signal spectrally spreaded on the basis of a code row obtained by time inversion of a specific code row allocated to each end device is inputted to the terminal 3-1 so that only the data allocated to each end device can be demodulated from the reception spread-spectrum signal selectively. In this case, the frequency of the output demodulation signal is twice as high as the frequency of the input signal.
Although the SAW signal processing apparatus shown in FIG. 1 has been already used for demodulation in spread-spectrum communication for special purposes, the apparatus has a large disadvantage as follows. Because the nonlinear effect of SAW is substantially small, the level of the output signal is low. Further, a replica signal spectrally spread on the basis of a code row obtained by time inversion of a specific code row allocated to each end device needs to be generated in the end device. Therefore, a replica signal generating circuit needs to be provided separately. There arises a disadvantage that the total circuit scale becomes large. Further, because the frequency of the demodulation signal is twice as high as the frequency of the input signal, it is difficult to handle the demodulation signal after that.